1. Technical Field
Embodiments of the present invention relate to a virtual image display apparatus which provides video formed by an image display element or the like to an observer, and in particular, to a virtual image display apparatus which is suitable for a head-mounted display mounted to the head of the observer.
2. Related Art
Various optical systems have been proposed as an optical system which is assembled in a virtual image display apparatus, such as a head-mounted display (hereinafter, referred to as an HMD) mounted to the head of an observer (see Japanese Patent Nos. 2746697, 3787399, 4218553, and 4819532).
For the virtual image display apparatus, such as an HMD, it is desirable to enlarge a viewing angle of video light and to reduce the weight of the apparatus. In particular, in order to improve the fit, it is important to reduce the thickness in a visual axis direction of the observer and to bring the center of gravity close to the observer.
If the field of view of the observer is completely covered and thus only video light is viewed, an external scene is not noticed by the observer, causing uneasiness for the observer. Furthermore, the external scene and video are displayed in an overlapping manner, various new usages, such as virtual reality, are produced. For this reason, there is a need for a display which displays video light in an overlapping manner without obstructing the field of view of the outside.
In order to improve the fit for the observer and to improve the appearance of the form, it is desirable to arrange a video display device at the side of the face, instead of being disposed above the eyes.
In order to reduce the size of the optical system and to position the video display device according to the position of the eyes of the observer not to obstruct the field of view, it is preferable to image display image light on the optical system once to form an intermediate image, and a relay optical system which enlarges the intermediate image to be displayed.
For example, Japanese Patent No. 2746697 proposes a relay optical system which uses a parallel planar light guide plate, in which a parabolic mirror is provided at an end surface thereof, and a projection lens, and forms an intermediate image inside the light guide plate. However, in the case of the optical system of Japanese Patent No. 2746697, the projection lens is large in size, obstructing reduction in size and weight.
Japanese Patent No. 3787399 proposes a relay optical system which uses a light guide member having a curved emission and reflection surface and a projection lens. However, the optical system of Japanese Patent No. 3787399 has no consideration of displaying an external scene to the observer. In order to display the external scene by applying the optical system, it is necessary to attach a compensation member to a reflection surface extending throughout the light guide member and to provide a half mirror on the bonding surface. However, since video light is reflected by the half mirror surface twice, video becomes very dark.
Japanese Patent No. 4218553 proposes a relay optical system which has a projection lens, a concave mirror, and a light guide plate. In this optical system, a wavelength plate and a polarization half mirror are combined, thereby enhancing reflection efficiency. However, in order to display an external scene by applying the optical system of Japanese Patent No. 4218553, it is necessary to attach a compensation lens to the concave mirror, resulting in an increase in thickness as a whole.
Japanese Patent No. 4819532 proposes a relay optical system which has a short total length and a compact arrangement due to a bent optical path. However, in the case of the optical system of Japanese Patent No. 4819532, a convex portion at the center of the light guide member or a projection lens obstructs the field of view. Furthermore, since video light passes through a half mirror, is then returned and reflected by the half mirror, and enters the eyes, video to be observed is darkened.